1. Industrial Field of Use
The invention relates to a filtering apparatus for fluids, including gases and liquids. More particularly but not exclusively, it relates to a filtering apparatus for hot gases.
2. Description of the Related Art
In many applications, a filtering apparatus for fluids comprises a plurality of filter elements arranged in a vessel. A fluid is fed into the vessel and is filtered through the filter elements before leaving the vessel through an outlet. An example of such a filtering apparatus is a hot gas filtering apparatus.
Hot gas filtering apparatuses are used to separate particulates from a hot gas flow which may have a temperature between 300.degree. and 1000.degree. C. and a pressure of up to 20 bars. Such hot gases are found, for example, in power generation systems where the gas drives a turbine. The gas may be derived from a process such as a pressurized fluidized bed. Hot gases of this kind are also found in gasification combined cycles and fluidized catalytic cracking.
In a process of this kind, the hot gases generated are laden with particles such as dust and ash. For this reason, after generation, the hot gases are generally passed first through a cyclone, which removes the majority of these particles. However, such cyclones do not remove fine particles such as fly ash and it is essential that such particles be removed since gas turbines, for example, are very sensitive to the presence of such particles. In addition, since the gas is exhausted to the atmosphere after passing through the turbine, it is desirable not to discharge such particles into the atmosphere. Hot gas filtering apparatuses are used to remove these particles.
A hot gas filtering apparatus may contain a number of filter elements which, because of the temperatures encountered (in the range of 800.degree. C.) are usually made of ceramic materials such as silicon carbide. However, such filter elements, commonly referred to as candles, have a low flow per unit pressure drop and this, combined with the requirement to remove all particles for significant periods of time and combined with high gas flow rates, requires a large number of filter elements. For example, a hot gas filtering apparatus may contain thousands of such elements.
One form of hot gas filtering apparatus has a filter vessel containing a plurality of filter elements with an inlet feeding particulate-laden hot gas to the outer surface of the filter elements and an outlet removing filtered hot gas from the interiors of the filter elements. The filter elements may be vertically suspended from a manifold.
Hot gas ceramic filter elements have a limited lifetime in the range of 6-18 months. Due to severe service conditions, they can be susceptible to damage. For example, ceramic candles are susceptible to thermal shock cracking, since ceramics are inherently brittle. For this reason, it is desirable to be able to remove and replace individual filter elements. However, in a conventional filtering apparatus, doing so can require the dismantling of significant portions of the filter vessel and is therefore time-consuming. Furthermore, service personnel who replace the filter elements frequently must enter into the region of the vessel surrounding the filter elements and may be exposed to harmful substances present in the vessel.
A filtering apparatus for use with a commercial-size pressurized fluidized bed combustion system may have as many as 10,000 filter elements. If the filter elements are arranged in a conventional manner within a vessel, the vessel and a tube sheet for supporting the filter elements must be quite large and correspondingly expensive to manufacture, especially if they are made from high temperature alloys. For example, a conventional flat tube sheet able to support 2000 ceramic candles may need a diameter of more than 5 meters. The large size of the tube sheet results in increased mechanical loading on the tube sheet from both the weight of the filter elements and the pressure differential across the tube sheet. This results in thicker and more expensive tube sheets as the number of filter elements increases. The increased expense becomes particularly marked as the operating temperature of the filtering apparatus increases, because the high operating temperatures and corrosive atmospheres in many applications require the use of high cost materials. Even these materials have significantly reduced strength and elevated temperatures, resulting in even thicker tube sheets being required.
Accordingly, there is a need for a filtering apparatus which can support a large number of filter elements with a lightweight, economical structure. There is also a need for a filtering apparatus which can compactly support a large number of filter elements while still permitting the filter elements to be easily replaced.
After filtering for a period of time, the filter elements of some fluid filtering apparatuses are customarily cleaned by supplying them with fluid which is directed into the interior of each filter element and emerges from the exterior surface to remove from the exterior surface particulate matter clinging to the surface. This cleaning method is commonly referred to as backwashing (when the fluid is a liquid) or blowback (when the fluid is a gas). The term "reverse flow cleaning" will be used to cover both situations. In filtering apparatuses in which blowback is performed, it is customary to blow gas through each filter element individually, and significant amounts of piping are required to conduct fluid to each filter element. When the filtering apparatus includes thousands of filter elements, the cost of the piping can be extremely high.